The invention relates to a process for the crosslinking of polyacrylates by means of electron beams.
Adhesives are products which, in accordance with their respective chemical composition and prevailing physical state at the time of application, permit wetting of the surface on the adherends or in the course of heating thereof (e.g., adhesive films) and, in the joint, form the adhesive film necessary for the transfer of force between the adherends.
An important criterion in producing an adhesive and for its properties is the degree of crosslinking, which affects both the viscosity and the adhesive properties. Crosslinking is the term used to denote the reaction in which a large number of linear or branched macromolecules, which initially are still soluble, become linked together to form three-dimensional polymeric networks (crosslinked polymers, network polymers) which are insoluble and now only swellable. Crosslinking is possible as a result of the formation of covalent and noncovalent (coordinative, ionic, physical, saltlike) bonds. Crosslinking can be carried out during the actual construction of the macromolecules and/or by reaction on preformed (pre)polymers which generally contain functional groups.
Within the industry, hotmelt processes with solventless coating technology are of increasing importance for the production of pressure-sensitive adhesive (PSA) compositions. Generally speaking, environmental regulations and increasing costs are pushing forward this development process. As well as SIS systems (styrene/isoprene/styrene copolymers), acrylic polymers are increasingly being applied from the melt as a polymer layer on backing materials.
As a result of the changeover to hotmelt processes, some of the crosslinking mechanisms employed to date are becoming superfluous. For example, thermal crosslinking techniques with metal chelates or polyfunctional isocyanates, which are very popular with the solvent systems, can no longer be used. At present, therefore, the crosslinking of polyacrylate PSA compositions by irradiation with ultraviolet light (UV) or with electron beams (EB)xe2x80x94the latter being known as electron beam curing (EBC)xe2x80x94is being implemented more intensively.
Generally speaking, UV crosslinking is a very simple process requiring only a simple coating used with a few low-pressure Hg lamps. UV crosslinking functions very well for polyacrylate compositions with low film thicknesses. The EBC technology is more expensive in terms of apparatus but tolerates the crosslinking of greater film thicknesses and faster web speeds.
For industrial applications, especially in automobile construction, there is a need for acrylic PSA tapes which possess thermal stability and shear strength. In addition to the high shear strength, however, the adhesive tapes ought also to possess high bond strength and a high contact adhesion (tack). In polyacrylate PSA compositions, high shear strength is achieved through a high average molecular weight and a high polarity. Furthermore, crosslinking must be carried out efficiently in order to produce an adhesive composition of very high shear strength.
The polar PSA compositions of very high shear strength possess the disadvantage, however, that they are unsuited to the hotmelt extrusion process, since high application temperatures are required and the molecular weight is reduced as a result of shearing in the extruder. Because of the high polarity and the high molecular weight, the bond strength of these adhesive compositions is not great and their tack is relatively low.
It is an object of the invention to provide a process with which it is possible to prepare a polyacrylate, especially an acrylic PSA composition, with good processing properties of the starting materials and intermediates for the hotmelt technology, for example, and combining ultimate shear strength with good bond strength and good tack.
The invention described presents a process for the selective preparation of structured polyacrylates and is outstandingly suitable for achieving the abovementioned object.
By selective irradiation of the polyacrylates with electron beams, segments of different hardness are produced; the resultant structuring of the crosslinked polyacrylates makes it possible to influence their properties in the desired manner.
The invention accordingly provides a process for the crosslinking of polyacrylate compositions, in which, by selective irradiation of the pressure-sensitive adhesive composition with electron beams, the curing of the polymer takes place only in certain structures and it is thereby possible to prepare structured pressure-sensitive adhesive compositions.